1. Field of the Invention
The present invention relates to a method for producing an electrolytic capacitor including an electrode (anode) formed of a porous body of a valve metal such as aluminum and tantalum. In particular, the present invention relates to a method for producing an electrolytic capacitor including a conductive polymer layer as the other electrode (cathode) corresponding to the electrode (anode) formed of the valve metal.
2. Description of the Prior Art
Conventionally, a one-solution method is known as a method for forming a conductive polymer layer for an electrolytic capacitor. In the one-solution method, a mixture of a monomer solution and an oxidizing solution is introduced on an oxide film formed on a surface of a porous body of a valve metal. However, in the one-solution method, immediately after the monomer and the oxidant are mixed, a reaction therebetween (oxidation polymerization) is initiated. Therefore, the activity of the mixed solution is reduced before the mixed solution reaches the depth of the pores of the porous body. This tendency is significant when a porous body having complex pores, such as sintered tantalum, is used. Therefore, the one-solution method is disadvantageous in the formation of a conductive polymer in the depth of the pores of the porous body.
Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 6-310380 discloses a one-solution method of introducing a mixed solution of a monomer and an oxidant onto an oxide film of a porous body at a low temperature, and then raising the temperature of the porous body or the mixed solution. According to this method, the amount of the conductive polymers formed in the vicinity of the center of the porous body may be equalized with that in the vicinity of the outer surface of the porous body to a better extent than the conventional one-solution method. However, in the one-solution method, the reaction cannot be stopped completely even if the monomer solution and the oxidizing solution are mixed in a low temperature. This results in an increase in the consumption of raw materials.
Therefore, generally, a two-solution method is performed. More specifically, a monomer solution and an oxidizing solution are prepared separately without being mixed with each other. Then, a porous body is dipped in the monomer solution and the oxidizing solution alternately so that a conductive polymer is formed in the pores of the porous body.
However, in the two-solution method, the solution in which the porous body is first dipped diffuses into the solution in which the porous body is secondly dipped, and thus a reaction occurs. This reaction occurring outside the porous body causes the yield of the conductive polymer adhering to the porous body to be low.
For example, Japanese Laid-Open Patent Publication (Tokkai-Hei) No.6-29159 discloses the following method. For example, a porous body is dipped in an oxidizing solution and a monomer solution in this order for 5 minutes each, and then taken out from the solutions. Thereafter, the porous body is allowed to stand in the air for 30 minutes during which polymerization of the monomer proceeds.
In this method, a large amount of the oxidant introduced into the porous body flows into the monomer solution so as to contaminate the monomer solution. In addition, the yield of the conductive polymer is not high.
Japanese Laid-Open Patent Publication (Tokkai-Hei) No.7-130579 discloses the following method. For example, a porous body is dipped in a monomer solution (aniline solution) for 30 seconds, allowed to stand at room temperature in the air for 10 minutes, dipped in an oxidizing solution for 30 seconds, and allowed to stand at room temperature in the air for 30 minutes to cause polymerization.
According to this method, the solvent in the monomer solution introduced first is volatilized so as to concentrate the monomer solution, and then the porous body is dipped in the oxidizing solution. Therefore, the outflow of the monomer solution can be suppressed. However, the above step cannot to applied to a monomer that is volatilized readily.
The two-solution method also poses the problem that a conductive polymer is hardly formed on an oxide film in the deeper portion of a porous body. In particular, when a monomer having a high polymerization rate such as pyrrole is used, a reaction proceeds rapidly when a porous body that has been dipped in one solution is dipped in the other solution. This rapid reaction makes it difficult to form a conductive polymer on an oxide film in the depth of the pores of a porous body. In order to coat the internal portion of the porous body with a conductive polymer layer so as to obtain full inherent capacitance, a large number of repetitions of a series of steps are required. In some cases, the conductive polymer may block the pores in the vicinity of the outer surface of the porous body, so that the conductive polymer cannot be formed in the internal portion of the porous body.
In a solid electrolytic capacitor, when the amount of the conductive polymer formed in the vicinity of the center of the porous body is smaller than that in the vicinity of the outer surface of the porous body, the ratio of the current capacitance to the expected capacitance based on the entire surface area of the porous body (hereinafter, referred to as "capacitance achievement rate") becomes low. In addition, even if the conductive polymer could be formed uniformly in the vicinity of the center of the porous body and in the vicinity of the outer surface thereof, there may be a problem as follows. When the total amount of the conductive polymer formed in the porous body as a whole is small, the obtained electrolytic capacitor has a large resistance and a large loss as a whole.